1. Field of the Invention
The field of the invention is digital controllers for controlling a machine, an assembly line or a commercial process. More particularly, the invention relates to motion control I/O modules for controlling motion along one or more axes.
2. Description of the Background Art
Programmable controllers are employed in many industrial and commercial applications to control the operation of various machines and processes. They fall into the broad category of real-time sampled data systems. In a certain maximum time, inputs are read, controlling equations are applied, and actuator outputs are provided. Besides controlling switching devices, these controllers also control closed loop servo devices by generating analog output signals and by reading encoded feedback signals and comparing actual position to desired position.
Controllers generally have a modular construction with some type of system processor module and a number of I/O modules for connecting to the I/O devices on a controlled machine or process. The modules are often supported in a chassis and plugged into a backplane circuit board for electrical interconnection and communication.
It is typical to provide an isolation interface in I/O modules to physically and electrically isolate 120-volt AC signals or 24-volt DC signals, for example, on the machine side of the controller, from the 5-volt logic-level signals within the controller electronics. This electrical isolation is typically accomplished with optical coupling circuits. With optical coupling, an overcurrent or fault on the machine side of the I/O module is isolated from the controller electronics.
Techniques for utilizing isolation interfaces in I/O modules have been previously disclosed in Maskovyak et al., U.S. Pat. No. 4,771,403, issued Sep. 13, 1988, and in Pepera et al., U.S. Ser. No. 07/580,716, both of which were assigned to the assignee of the present invention. In Maskovyak et al., a pair of semi-custom integrated circuits are connected on opposite sides of the isolation interface and are set to operate in one of four jumper-selectable modes. Specific circuitry is enabled within the semi-custom integrated circuit according to which mode of operation was selected. This provided a limited number of different modes of operation. Other changes in operation would require re-engineering of the semi-custom integrated circuits.